This invention is concerned with the measurement of pressure within sealed containers.
A variety of techniques using acoustic energy to determine the characteristics of a gas or gas mixture sealed within a closed vessel have been disclosed in the prior art. Brown, U.S. Pat. No. 3,942,381, measures the time required from an ultrasonic pulse to propagate across a pressure vessel, echo off the far side, and return. The travel time is related to the pressure of the gas in the container, since this time varies with the velocity of sound in the gas, and the velocity in turn is sensitive to the pressure of the gas. In Shibasaki, U.S. Pat. No. 4,187,718, and Miyahara, U.S. Pat. No. 4,406,157, the internal pressure of a sealed container is determined by measuring the dampening characteristic of a vibrating container wall using a self-correlation function. Parker, U.S. Pat. No. 4,474,061, measures the volume of gas in a container by creating cyclic pressure waves in the gas and detecting the waves with a transducer. This approach requires transducers to be mounted inside the vessel. Terhune, U.S. Pat. No. 4,520,654, determines the concentrations of various gases in a volume of gas by propagating an ultrasonic pulse through the gas. The detected attenuation and velocity of the pulse are compared to a reference pulse in a standard gas mixture in order to estimate the composition of the unknown gas.
In spite of these contributions to the prior art, a continuing need exists for a technique to measure the pressure of a gas within a sealed container. Techniques are needed in which the sonic or ultrasonic wave carrying the information is not overshadowed by other waves traveling in the container. Many applications exist for use with high pressure containers that have walls which may be too stiff for some prior art methods, and for measuring gas pressure entirely from outside the sealed container.